Transmission Electron Microscopy is a key technique to characterize nanoparticles and nanostructures, and to study their interactions with cells and tissues. At CNI, we apply advanced transmission electron microscopy techniques across the areas of materials science, nanotoxicology and biology, and we strive to develop novel methods and to extend the range of applications. Specifically, we are developing electron diffraction methods based on the precession of the electron beam. The precession electron diffraction unit mounted on our Transmission Electron Microscope (Zeiss Libra 120 Plus), enables us to use the TEM as a single crystal diffractometer for structure solution of micro- and nanocrystals. Precession electron diffraction is a way of collecting diffraction patterns in which the electron beam precesses on a cone with a vertex fixed on the specimen plane. During the precession movement the pattern is tilted off the zone axis, with few reflectionss excited at the same time, and the dynamical interactions between the diffracted beams are strongly reduced. The resulting pattern is quasi-kinematical ("x-ray like"), and the diffractions intensities extracted from these patterns enable crystal structure solution (Fig. 1).

Fig.1: Electron diffraction pattern of C12Al14O33 (cubic a=12Å), oriented in [111] zone axis, taken without precession (left) and with precession (precession angle 3.5°) (center). For comparison a kinematical simulation is displayed at the right.

Precession electron diffraction combined with a data collection of a tilt series of diffraction pattern (one every 1°) can give a full coverage of the reciprocal space, which can be reconstructed in 3D tomography (Fig. 2).

Fig.2: 3D reconstruction of the reciprocal space of Zn4Si2O7(OH)2·H2O obtained from a series of 121 diffraction patterns taken with precession on, with a 1° precession angle. The patterns are collected every 1°over a tilt range of +60°, -60°.

Intensity data extracted from this data collection are also suitable for structure solution, since they are recorded in random crystal orientations with few spots excited at the same time.

The precession unit installed in our laboratory represents a unique facility in Italy, and will be devoted to structure solution of new crystalline organic compounds and pharmaceuticals. We are actively seeking industrial and academic partners to extend the use of this facility to a range of materials, and we collaborate with Nanomegas company, the manufacturer of the Digistar device for precession electron diffraction, for further technical developments of this promising technique